why do programs working in H264 make the same information come out in different file sizes?

Ok, nubbie to codec info, so I'm looking for help understanding something, please.

I have taken an mov, H264 file in Final Cut Pro, shifted the sound track slightly, and exported the file.
The orig file was around 2 gig.  This new file comes out 4-5 gig.

The reversed has happened in After Effects, where the file was exported and came out 1/2 to 1/3 the size of the original.

In both cases, the file was an mov, H264 codec.  If both are using the same codec, which I take to mean the code that's used to provide the image data, shouldn't the file sizes be close to the same size?  If not, what's missing or added to account for the differences in file size?

As I said, I'm new to this and looking for clearer explanations than the normal, general.

Thanks.

It is the same formats, but not the same information. There are several encoding parameters that control the amount of compression. Higher compression means lower quality and smaller filesize.

You should be able to configure the encoding settings somewhere. Look for the keywords "bitrate" and "quantinizer". Higher bitrate = lower compression. Higher quantinizer = higher compression.

Thank you! This is the kind of answer I appreciate.

Cheers.